Field of the Invention
The invention relates to a self-ignition control in a four-stroke internal-combustion engine.
It is already known from EP-A-0,863,301 to control the intake and exhaust valves of the cylinders of a four-stroke engine so as to hold back in the cylinders a certain amount of burnt gases prior to allowing fresh gas to pass into the cylinders, by controlling the amount of burnt gases kept therein and by minimizing mixing thereof with fresh gas, so that self-ignition of the fresh gases is initiated in the combustion chambers by the burnt gases which are at a high temperature and contain active species. The open period of the exhaust valves of the cylinders can therefore be reduced. In a variant, the exhaust valves can remain open for a longer time so as to reintroduce burnt gases into the cylinders when the pistons are displaced from the top dead center to the bottom dead center thereof, and opening of the intake valves is delayed accordingly.
These particular controls of the intake and exhaust valves are used during part-load operation of the engine with controlled self-ignition, which leads to a substantial reduction in the discharge of nitrogen oxides to the atmosphere and in the consumption whereas, at high load, one reverts to a conventional control of the valves and to spark ignition.
The present invention applies self-ignition techniques to a four-stroke engine equipped with several exhaust valves and several intake valves per cylinder, by means of an exhaust and intake valve control process that is simple, effective and easy to implement.
The invention is a process using self-ignition control in a four-stroke internal-combustion engine, wherein exhaust means of the cylinders are controlled so that the cylinders are filled partly with burnt gases and partly with fresh gases for the compression phases when the engine runs at partial load. Each cylinder is equipped with at least two exhaust means such as valves. Opening and closing each one of the two exhaust means of a cylinder are controlled differently so as to ensure discharge of the burnt gases from the cylinder during the exhaust phase and reintroduction of burnt gases into the cylinder during the intake phase when the piston is displaced from the top dead center to the bottom dead center thereof.
The process according to the invention therefore differentiates and dissociates the functions of the two exhaust means of each cylinder, so as to obtain a large amount of burnt gases in the cylinder, and easier control of the opening and closing cycles.
In an embodiment of the invention, a first exhaust means of the cylinder is open and the other is closed during the exhaust phase when the piston is displaced from the bottom dead center to the top dead center thereof, then the first exhaust means is closed and the other is open at the beginning of the intake phase when the piston is displaced from the top dead center to the bottom dead center thereof.
In a variant, the two exhaust means of the cylinder can be open during the exhaust phase when the piston is displaced from the bottom dead center to the top dead center thereof, then one of the exhaust means is closed and the other is kept open at the beginning of the intake phase when the piston is displaced from the top dead center to the bottom dead center thereof.
Furthermore, reintroduction of burnt gases into the cylinder during the intake phase and intake of fresh gas into the cylinder can take place almost at the same time, which saves changing intake means control.
In a variant, there is a lag between reintroduction of burnt gases in the cylinder and intake of fresh gas in the cylinder, reintroduction of burnt gases in the cylinder starting before intake of fresh gases.
Mixing of the burnt gases and of the fresh gases in the cylinder is thus reduced, which can favor self-ignition of fresh gases in the cylinder under certain operating conditions.
Reintroduction of the burnt gases into the cylinder preferably ends with closing of the exhaust means left open, before the end of the fresh gas intake in the cylinder.
When the engine is equipped with several intake means per cylinder, the fresh gases can be allowed to pass into the cylinder through only one intake means.
This intake means is preferably arranged directly opposite the exhaust means that is in closed position during the intake phase, in cases where the operating conditions impose limitation of fresh gas and burnt gas mixing. In the opposite case, the intake means is arranged directly opposite the exhaust means that is in the open position during the intake phase, in order to favor mixing of the gases.
In another variant, both intake means are used, notably to favor mixing of the gases.
More generally, the process according to the invention allows reintroduction into a cylinder a sufficient amount of burnt gases to initiate self-ignition of the fresh gases allowed to pass into the cylinder thereafter. Discharge of the burnt gases from the cylinder during the exhaust phase and reintroduction thereof into the cylinder during the intake phase allows saving recompressing the burnt gases in the cylinder when the piston is displaced from the bottom dead center to the top dead center thereof during the exhaust phase, which would lead to thermal losses and increase the work of the low-pressure loop.